In the past recent years, more and more tanks, vessels and piping for the chemical industry have been made of fibreglass reinforced polyester. The success of these structures and, more recently, those made of dual laminate ccnstructicn consisting of an outer fibreglass reinforced polyester shell and an inner corrosion-resistant lining has depended to a great extent not only on the chemical resistance of the inner lining, but also on the ability of this inner lining to act as an effective vapour-liquid barrier. In other words, eventhough the fibreglass reinforced polyester forming the outer structure itself may be resistant to the chemical contained within the vessel, voids in the inner corrosion resistant lining may still allow the chemical contents to pass through this inner barrier and weaken the structural shell. At present, when the chemical contents do pass through the inner lining, it may take a period of months or years before it is apparent that failure of the lining has occured. In some instances, the first indication of such failure is a catastrophic leakage through the structural shell of the vessel.
It is known to use a metal foil, such as aluminum sheet, as a conducting layer between the inner lining and the outer structural shell to detect failures in the inner lining of the vessel. A failure will give rise to an electric path between the chemical, in most cases an electro-conductive fluid, and the conducting layer, allowing to easily detect the loss of integrity in the inner lining simply by monitoring the electrical impedance between the chemical and the conducting layer. However, the use of metal foils has serious disadvantages in applications where the contents of the vessel is a very corrosive material such as an inorganic acid. Should this acid leak through the inner lining of the vessel, it will rapidly and preferentially attack the metal foil and leave large areas of the inner lining no longer bonded to the structural outer layer of the vessel. A foil of a metal with similar resistance to PVC would be very expensive and rather defeat the object of using a plastic vessel for containing corrosive chemicals.
To overcome this drawback, it has been suggested to replace the metallic foil by a conducting or semiconducting layer incorporating finely divided graphite, carbon or metal particles contained in a glass mat. Such layer is highly resistant to corrosion capable to withstand exposure to various chemicals during long time periods.
A major difficulty with prior art leak detection systems resides in the inability of such systems to identify the position of the fault in the inner liner. When an alarm condition is generated, indicating loss of integrity of the inner liner, the entire vessel must be inspected in order to locate the fault. Minute cracks or punctures may be extremely difficult and time consuming to locate, especially in large vessels considerably increasing the vessel downtime.